Synthesis gas (hereinafter referred to as syngas) is a mixture of hydrogen (H2) and carbon monoxide (CO). Syngas can be produced, in principle, from virtually any material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials. It is preferable to utilize a renewable resource to produce syngas because of the rising economic, environmental, and social costs associated with fossil resources.
Syngas is a platform intermediate in the chemical and biorefining industries and has a vast number of uses. Syngas can be converted into alkanes, olefins, oxygenates, and alcohols. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can be converted to liquid fuels, for example, by methanol synthesis, mixed-alcohol synthesis, Fischer-Tropsch chemistry, and syngas fermentation to ethanol. Syngas can also be directly combusted to produce heat and power.
It is known that certain microorganisms can ferment combinations of carbon monoxide, hydrogen, and carbon dioxide to ethanol according to the following overall reactions:6CO+3H2O→C2H5OH+4 CO2 6H2+2CO2→C2H5OH+3H2O
Fermentation according to these reactions often employs anaerobic conditions. Depending on the organism and reaction conditions (e.g., pH), various other products can be produced, such as acetic acid, butyric acid, or butanol. Some strains of anaerobic microorganisms are reported to convert syngas to ethanol, n-butanol, or other chemicals with high selectivity.
Syngas fermentation to products such as ethanol and acetic acid can achieve fairly high selectivity, but due to mass-transfer limitations and low activities per unit volume of reactors, the reactors tend to be very large. Syngas conversion in well-mixed reactors is generally limited.
Additionally, production of ethanol from syngas will result in the co-formation of CO2. This CO2 is present in the tail gas of the fermentor, i.e. a vapor stream deriving from the fermentor. The tail gas generally contains any unconverted syngas, the CO2 produced in the fermentation, and the inerts (including CO2) initially contained in the syngas feed to the fermentor. The tail gas is commonly burned to recover the energy in the unconverted syngas as well as the energy in any other combustible components contained in the conditioned syngas stream, such as methane.
The unconverted syngas cannot simply be recycled to extinction. The inerts and the CO2 produced in the fermentor must be removed from the overall process. Removal of CO2 from the tail gas in a separate unit downstream of the fermentor is relatively expensive. Also, separation of the unconverted syngas from the inert gases and other species (such as methane) is not desirable.
In view of these problems associated with syngas fermentation, what is needed is an improved process configuration that more efficiently utilized syngas components to produce liquid products of interest, such as ethanol. Preferably, any such improvements do not cause significant increases in overall plant capital costs.